Electrolysis of water to obtain hydrogen and oxygen (and ozone) has conventionally been conducted extensively. For example, ozone for sterilization and other uses and other substances have been produced using an electrolytic cell equipped with a diaphragm and using an aqueous solution of caustic potash as the electrolyte.
In the production of ozone by the electrolysis of water, attention is focused on how to reduce the power unit or lower the production cost of the electrodes in order to conduct electrolysis efficiently. Recently, an electrolytic process using a solid electrolyte has been employed in which an ion-exchange membrane of the sulfonic group-containing fluoroplastic type, for example, is used as a diaphragm. The diaphragm is first coated on one side with an active anode material and is coated on the other side with an active cathode material, and thereafter, electrolysis is performed with the ion-exchange membrane as the solid electrolyte while water is fed from the anode side. In this electrolytic process, in which electrolysis is generally conducted with the electrodes being close to or in close contact with the diaphragm, the sulfonic groups and fluorine compound (fluoroplastic) in the diaphragm function as co-catalysts for ozone evolution. Therefore, only those parts of the electrode material (e.g., lead dioxide) which are located near the sulfonic groups, i.e., close to the ion-exchange membrane, greatly contribute to ozone evolution.
This does not cause any particular problems if the electrolyte is sufficiently pure, i.e., having an electrical conductivity of about 1 .mu.S/cm or less, provided that the parts of the electrode material other than those in direct contact with the ion-exchange membrane are insulated and, hence, do not contribute to electrolysis. In this case, the electrode material is utilized only partly, and the electrode production cost, which is relatively high, cannot be reduced. In practice, however, electrolysis also takes place on the parts of the electrode which are not in direct contact with the ion-exchange membrane, because the electrical conductivity of the electrolyte in contact with or close to the ion-exchange membrane usually increases to 10 .mu.S/cm or more due to carbonic acid gas present in the electrolyte and substances dissolved out from the electrode. Since electrolysis on those parts of the electrode material proceeds in the absence of a co-catalyst, a problem occurs in that such electrolysis of water results mainly in the evolution of oxygen, thus lowering the ozone-evolving efficiency and current efficiency for the whole electrolytic process.
JP-A-63-100190 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example, discloses an electrode structure in which the anode is a two-layer structure and contains an electrode material only in the layer in contact with an ion-exchange membrane (solid electrolyte), to improve electric current efficiency. However, since such an electrode structure has a limited practical electrode area, the current density is increased, resulting in a possible shortening of the electrode life. In addition, there is another problem in that the electrolytic cell necessarily has a complicated structure.